Technical Field
The technical field relates to fuel injection for internal combustion (IC) engines, and more particularly to detecting fuel injector quantity drift on diesel-cycle IC engines.
Background to the Technical Field
Diesel engines have historically used various forms of fuel injection. Contemporary diesel engine fuel injection systems are usually “common rail” systems. In a common rail fuel injection system a high-pressure pump stores fuel in a reservoir or accumulator at high pressure. The term “common rail” is a reference to the fact that all of the fuel injectors for the engine are supplied from this single high pressure accumulator. The fuel injectors are usually controlled by a programmable micro-controller based device referred to as an engine control unit (ECU). With the fuel injectors electrically activated by the ECU, fuel is sprayed into the cylinders at the desired pressure. Since the fuel pressure energy is stored remotely and the injectors are electrically actuated, the injection pressure at the start and end of injection should remain very near the pressure in the accumulator (rail) and produce an injection pulse with close to a square wave form. If the accumulator and injectors are functioning properly, injection pressure and rate will be the same for each of the multiple injection events during a combustion stroke for a cylinder and duration of the pulse will conform to target values. Under these conditions the quantity of fuel delivered highly controllable.
Common rail fuel injection systems have made it easier to control diesel engine exhaust emissions and noise through modification of combustion stroke fuel injection patterns. While the quantity of fuel to be injected per power stroke is fixed by an externally sourced demand for power, such as the position of a vehicle accelerator pedal, delivery of the fuel is distributed over multiple injection pulses. A minimum multiple injection pulse pattern includes two injection pulses. These are referred to as a pilot injection pulse and a main injection pulse. A pilot injection pulse precedes top dead center (TDC) of piston travel at the end of the compression stroke by a few degrees while a main injection pulse occurs close to TDC. Dispersing fuel among multiple points spreads out the burn and mitigates the characteristic of diesel engines of combusting most of the injected fuel almost immediately upon initial ignition. This reduces the noise and can be tailored to reduce emissions.
The quantity of fuel actually injected by an injector during an injection pulse is subject to drift away from nominal values over the service life of an engine. When the fuel to be injected for a given power stroke is divided among more than one injection pulse, the effects of injector drift for that cylinder tend to be scaled by the number of pulses used in the injection pattern. This can have a number of consequences including an increase in engine emissions.
Diesel engine on board diagnostic (OBD) systems has provided monitoring of fuel system performance to detect malfunctions in individual injector fuel delivery. One way this has been done is by comparing changes in crankshaft speeds for each cylinder's power stroke. A significant speed difference from the average may indicate a correlated injector's fuel delivery was off nominal Another form of monitoring is an intrusive test that attempts to inject a small quantity of fuel and view the subsequent crankshaft acceleration. If the acceleration was outside an expected range a fault in an injector could be indicated. Any diagnostic routine should take into account that injector drift may differ from injector to injector on the same engine.